Mean total flux of an ensemble of fitted line profiles over a range of values of ALFALFA S/N. Only 5 of the THINGS galaxies are shown, as labelled. Construction of these Monte Carlo simulations is described in Sect. 3.2.2. The square symbols give the mean, the diamonds the median, and the vertical bars show the standard deviation for each ensemble. The half-tone lines give the values from the profile fits to the original THINGS data. Red X show the points at which the peak-to-rms S/N equals 2.